Natural gas reserves have been found in remote areas where it is uneconomical to develop the reserves due to the lack of local markets for the gas and the high cost of transporting the gas to distant markets. This high cost is often related to the extremely low temperatures needed to liquefy the highly volatile gas during transport. An alternative is to locally convert the natural gas to products that can be transported more cost effectively.
Natural gas comprises several components, including alkanes that are usually separated before being fed to different downstream processes. Alkanes are saturated hydrocarbons (compounds containing hydrogen [H] and carbon [C]) whose molecules contain carbon atoms linked together by single bonds. The simplest alkanes are methane (CH4), ethane (CH3CH3), and propane (CH3CH2CH3). Separating the different components of natural gas typically entails subjecting the natural gas to a very expensive, multi-stage process that is performed in a so-called gas plant. Inefficient separation can lead to lower product selectivity and catalyst deactivation resulting from the formation of coke deposits (i.e., low volatile hydrocarbonaceous substances) that plug the micropores and block the active sites of the catalyst. Therefore, a simpler, less expensive, and more efficient process is needed to separate natural gas into useful components.
One process that utilizes hydrocarbon compounds recovered from a gas plant is the oxidative dehydrogenation (ODH) of alkanes to olefins. In ODH, the alkanes are dehydrogenated in the presence of oxygen, typically in a short contact time reactor containing an ODH catalyst, thereby producing olefins. Olefins (also called alkenes) are unsaturated hydrocarbons whose molecules contain one or more pairs of carbon atoms linked together by a double bond. Generally, primary olefin molecules are commonly represented by the chemical formula CH2═CHR, where C is a carbon atom, H is a hydrogen atom, and R is an atom or pendant molecular group of varying composition. Another process using hydrocarbon feed to make carbon monoxide (CO) is the synthesis gas generation via catalytic partial oxidation or reforming reactions.
Olefins and CO are useful as a feedstock for various downstream processes, such as carbon filament production. In a conventional process, carbon filaments are grown during the thermal decomposition of olefins or CO in the presence of a metal catalyst. Producing carbon filaments in this manner depends upon an upstream process generating olefins or CO to supply the feed components for carbon filament growth. Carbon filaments produced from the catalytic decomposition of olefins or CO may have a wide variety of diameters (from tens of nanometers to tens of microns) and structures (e.g., twisted, straight, helical, branched, and mixtures thereof). Carbon filaments, especially when combined within a polymer matrix to form an engineered composite material, are known for their outstanding physical properties. However, the high cost of manufacturing carbon filaments continues to be an impediment to their widespread use. Thus, an ongoing need exists to develop efficient methods for producing carbon filaments. Using a process for converting alkanes directly to carbon filaments in accordance with the present invention eliminates the intermediate step of converting alkanes to olefins or CO, and thus avoids the high operating, equipment, and catalyst costs associated with production of olefins or CO.